In recent years, as a blue-violet semiconductor laser has been put into practical use, a Blu-ray disc (hereinafter, called as a BD) as a high-density and large-capacity optical information recording medium (hereinafter, also called as an optical disc) having the same size as a CD (Compact Disc) or a DVD (Digital Versatile Disc) has been put into practical use. The BD is an optical disc having a protection substrate of about 0.1 mm in thickness, and is used in recording or reproducing with use of a blue-violet laser light source of about 400 nm wavelength, and an objective lens having a numerical aperture (NA) as high as 0.85.
Similarly to the above, an HD DVD, which has a protection substrate of about 0.6 mm in thickness, and is used with a blue-violet laser light source of about 400 nm wavelength, and an objective lens having a numerical aperture of 0.65, has also been put into practical use. The optical discs to be recorded or reproduced using a blue-violet laser light source are generically called as high-density optical discs.
There has been proposed an optical head for recording or reproducing information on or from the high-density optical disc by collecting blue-violet laser light. A construction example of the optical head is shown in FIG. 17. Referring to FIG. 17, the reference numeral 101 indicates a light source for emitting blue-violet laser light, 102 indicates a diffraction grating, 103 indicates a polarized beam splitter as a light splitting element, 104 indicates a collimator lens, 105 indicates a quarter wave plate, 106 indicates an achromatic lens, 107 indicates an objective lens, 108 indicates a detecting lens, and 110 indicates a light receiving element. These parts constitute an optical head 130. The reference numeral 160 indicates a high-density optical disc having two layers of information recording surfaces.
In the following, described is an operation of the optical head 130 for recording or reproducing information on or from the optical disc 160. Blue-violet laser light emitted from the light source 101 is divided into a main beam as zero-order light and a sub beam as ±first-order diffracted light by the diffraction grating 102. The main beam and the sub beam are transmitted through the polarized beam splitter 103, and converted into substantially parallel light by the collimator lens 104. The main beam and the sub beam converted into the substantially parallel light are converted from linearly-polarized light into circularly-polarized light by the quarter wave plate 105, transmitted through the achromatic lens 106, and collected on an information recording surface of the optical disc 160 as a light spot through a protection substrate by the objective lens 107.
After reflected on the information recording surface of the optical disc 160, the main beam and the sub beam are transmitted through the objective lens 107 and the achromatic lens 106, converted into linearly-polarized light different from the incoming path by the quarter wave plate 105, transmitted through the collimator lens 104, and reflected on the polarized beam splitter 103. After being subjected to astigmatism by the detecting lens 108, the reflected main beam and sub beam are guided to the light receiving element 110, where an information signal and a servo signal are generated.
In this example, as a method for detecting a focus error signal, there is used e.g. a so-called astigmatism method, wherein astigmatism is imparted to laser light on the outgoing path by the detecting lens 108, and a focus error signal is obtained using a four-divided light receiving pattern. As a method for detecting a tracking error signal, there is used e.g. a so-called 3-beam method or differential push-pull method (DPP method) using a main beam and a sub beam generated by the diffraction grating 102.
There is known a drawback peculiar to an optical head for recording or reproducing on or from an optical disc, using blue-violet laser light of about 400 nm wavelength. Specifically, in the case where a resin is used as a material for an optical component constituting an optical system, absorption of blue-violet laser light is gradually increased in the interior of the optical component, with the result that the light transmittance of the resinous optical component is gradually lowered. For instance, a portion where blue-violet laser light is transmitted may turn yellow, or a surface of the light transmitted portion may become rough, with the result that the transmittance of the light transmitted portion with respect to laser light is lowered. The lowered transmittance of the optical component may degrade the S/N ratio of an information signal, a focus error signal, and a tracking error signal, which may lower reliability of a device incorporated with the optical head.
In view of the above, for instance, patent literature 1 discloses an optical head constituted of an optical component having a UV resistance. In the specification, the UV resistance means a property that no physical change occurs in a UV resistant member, even if the member is exposed to UV light for a long term. An example of the UV resistant member is an optical component made of silica glass, as a kind of a glass material. A detecting lens is also made of silica glass having a UV resistance.
Forming an optical component of a resin is advantageous in simplifying the arrangement of an optical head, and making the optical head lightweight and inexpensive. In view of this, a resin having a relatively high UV resistance is used for an optical component to be used at a site where the light flux diameter of blue-violet laser light to be transmitted or reflected is large, and the laser light amount per unit area is relatively small. For instance, an optical head incorporated with e.g. the collimator lens 104 or the achromatic lens 106 in FIG. 17, which is made of a resin, has been put into practical use.
In recording or reproducing on or from the optical disc 160 having plural information recording surfaces, reflected light from an information recording surface other than a targeted information recording surface to be used in information recording or reproducing is collected at a position different from the light receiving element 110. For instance, as shown in FIG. 18, in recording or reproducing on or from a first information recording surface of the optical disc 160, laser light L2 reflected on a second information recording surface, which is different from the first information recording surface to be used in information recording or reproducing, and is located at a rearward position, viewed from the side of the objective lens 107, is collected at a forward position of the light receiving element 110 with respect to laser light L1 reflected on the first information recording surface.
As described above, since the laser light flux diameter is small, and the laser light amount per unit area is exceedingly large at a position where laser light is collected, it is difficult to use a detecting lens made of a resin at the light collecting position. Further, in the case where information is recorded or reproduced on or from the optical disc 160 having plural information recording surfaces by the optical head using blue-violet laser light of about 400 nm wavelength, particularly, an intensity of reflected light from an information recording surface other than a targeted information recording surface to be used in information recording or reproducing is increased, and the laser light amount per unit area is increased. Thus, it has been actually impossible to use a detecting lens made of a resin in an optical head using blue-violet laser light.
Further, heretofore, there has been no description about an influence of reflected light from an information recording surface other than a targeted information recording surface to be used in information recording or reproducing, to the position of a detecting lens. Conventionally, a detecting lens has been made of a material other than a resin i.e. a glass material having a UV resistance. Thus, there is no disclosure about an arrangement of using a detecting lens made of a resin, in view of the above influence.
Furthermore, in the case where the detecting lens 108 is subjected to positional adjustment in the optical axis direction to correct an offset of a focus error signal resulting from e.g. a positional displacement between the light receiving element 110 and the light source 101, if the positional displacement is large, it is required to largely move the detecting lens 108. As a result, as shown in FIG. 18, reflected light from an information recording surface other than a targeted information recording surface to be used in information recording or reproducing is collected near the detecting lens 108 or inside the detecting lens 108 by the positional adjustment.
In the above arrangement, since the laser light amount per unit area at the light collecting position is exceedingly large, it is impossible to use a detecting lens made of a resin, as the detecting lens 108. However, the conventional arrangement has been made based on the premise that the detecting lens 108 is made of a glass material, and there has been no need of considering the above drawback. Thus, there has been made no investigation about an arrangement of using a detecting lens made of a resin, considering the positional adjustment.
Patent literature 1: JP2003-77163A